1. Field of the Invention
The present invention relates to electrical power supplies and, more specifically, to power supplies employing over-voltage protection devices.
2. Description of the Prior Art
Existing power supplies, such as those used in lithium ion battery chargers, often employ a mechanism in which a pair of inductors are used to source current from a DC source to a load (ie., the charger). Typically, a primary inductor stores energy from the DC source in the form of a magnetic field and a control circuit intermittently interrupts current flow through the primary inductor, thereby causing the magnetic field to collapse. A portion of the magnetic field collapses onto a secondary inductor that is electrically coupled to the load. The collapsing magnetic field induces a current in the secondary inductor, thereby supplying current to the load. A feedback circuit is used to indicate to control circuit when the secondary side voltage is below a predetermined level. The control circuit then causes an interruption of the current through the primary inductor, thereby causing a transfer of energy to the secondary inductor. Once the control circuit senses that the voltage on the secondary side is above the predetermined level, current flow through the primary inductor is allowed to resume, thereby storing energy in anticipation of a subsequent energy transfer.
The feedback circuit may fail in such a way that the control circuit never senses the secondary side voltage rising above the predetermined level. Thus, energy will continue to be transferred to the secondary side, thereby causing the secondary side voltage to rise above the normal operating voltage of the load. Such a rise in voltage may be undesirable in battery chargers for various reasons.
Therefore, there is a need for a circuit that prevents over-voltage conditions on a power supply secondary side upon the occurrence of a failure of a feedback circuit.